The Real Madrid goalkeeper experienced a tumultuous working relationship with the Portuguese boss but insists he has nothing personal against the newly appointed Chelsea manager
The 50-year-old, who was announced as Chelsea's new manager on Monday, kept the World Cup-winning shot-stopper on the bench for large periods of last season, preferring Diego Lopez in los Blancos' first team.
The Spain international was dropped from the squad altogether for Real Madrid's final Liga game of the season against Osasuna; Casillas claims he was fully fit to play but does not have any personal grievances with Mourinho.
"My back is fine. Was I fit to play? I'm always able to play, especially after such a long time out on the sidelines," the Spaniard explained to reporters.
Nevertheless, the 32-year-old is happy to let bygones be bygones and wished his former boss the best of luck in his new role at Stamford Bridge.
He continued: "I don’t bear any grudges against Mourinho, nor do I feel badly treated by him. He made decisions which he felt were the best ones for the club and I personally nothing personal against him.
"Unfortunately, we didn’t reach our objectives in the Copa del Rey and the Champions League, but I wish him the best of luck at Chelsea because he always stood up for Real Madrid."
Casillas' bit-part role in the Real Madrid season has left the goalkeeper lacking match fitness in the lead up to the Confederations Cup, but he is determined to prove himself for the national team once again.
"I really appreciate the confidence Del Bosque has placed in me and I promise to repay him by working as hard as I can," he added.
"I am at a slight disadvantage to my team-mates in terms of fitness and so I really need to get down to some hard work.